Rate and/or acceleration sensor

ABSTRACT

The invention is directed to a proof body such as a gyroscope in which the spin axis is constrained to the housing or vehicle by five pairs of orthogonally disposed wire pairs. The tension force each wire exerts on the gyroscope is sensed by determining the frequency of transverse vibration in each wire. The acceleration along three orthogonal axes and the rate of turn about two axes is obtained by processing the vibration-frequency data so as to solve the relationship: Tension K(frequency)2, for each wire. A tension control servo means is utilized for comparing the vibration frequency of the wire opposite a tension transducer with a precision reference frequency. The resulting error signal is used to activate the tension transducer so as to maintain a constant frequency (and tension) in the opposite wire under all acceleration and rate conditions. Consequently, the strain in the opposite wire of each pair will remain constant and the axes orientation of the gyroscope will remain fixed while the other wire of each pair will be vibrating at a frequency which will be proportional to the forces acting on the gyroscope. Means are provided for sensing this frequency and for providing an output signal which is proportional to the acting force. An accelerometer proof mass can be substituted for the gyroscope as a proof body with a resultant loss in rate sensing.

United States Patent [72] Inventors William M.Scarborough Whittier;Doyle E. Wilcox, Hacienda Heights, both of Calif.

21 Appl. No. 42,359

[22] Filed June 1, 1970 [45] Patented Dec. 28, 1971 [73] Assignee NorthAmerican Rockwell Corporation [54] RATE AND/OR ACCELERATION SENSORPrimary Examiner-Richard C. Queisser Assistant Examiner-HerbertGoldstein AttorneysL. Lee l-lumphries, H. Fredrick Hamann and EdwardDugas ABSTRACT: The invention is directed to a proof body such as agyroscope in which the spin axis is constrained to the housing orvehicle by five pairs of orthogonally disposed wire pairs. The tensionforce each wire exerts on the gyroscope is sensed by determining thefrequency of transverse vibration in each wire. The acceleration alongthree orthogonal axes and the rate of turn about two axes is obtained byprocessing the vibration-frequency data so as to solve the relationship:Tension K (frequency), for each wire. A tension control servo means isutilized for comparing the vibration frequency of the wire opposite atension transducer with a precision reference frequency. The resultingerror signal is used to activate the tension transducer so as tomaintain a constant frequency (and tension) in the opposite wire underall acceleration and rate conditions. Consequently, the strain in theopposite wire of each pair will remain constant and the axes orientationof the gyroscope will remain fixed while the other wire of each pairwill be vibrating at a frequency which will be proportional to theforces acting on the gyroscope.

Means are provided for sensing this frequency and for providing anoutput signal which is proportional to the acting force. Anaccelerometer proof mass can be substituted for the gyroscope as a proofbody with a resultant loss in rate sensing.

UTILIZATION KANS PATENTED B15828 IQTi i saw 2 OF 2.

.INVENTORS DOYLE E. WILCOX ATTORNEY RATE AND/OR ACCELERATION SENSORBACKGROUND OF THE INVENTION This invention relates generally to rateand/or acceleration sensors and, more particularly, to a supported proofbody for detecting rate and/or acceleration. A prior art patent ofinterest is U.S. Pat. No. 3,143,891, entitled Angular Accelerometer," byA. M. Voutas. In the system disclosed in this patent, the angulardisplacement detector is a vibrating member or pair of members whosenatural frequency varies in accordance with the torsional stress appliedthereto. The relative angular displacement of the casing of theaccelerometer with respect to the inertia mass causes a torsional stressto be applied to the vibrating member. The angle of displacement ismeasured by the change in natural frequency of the vibrating member, orthe difference in natural frequencies of a pair of oppositely affectedvibrating members.

Another patent of interest is U.S. Pat. No. 3,329,027, entitledAccelerometer," by Davidson et al. The vibrating wire accelerometer inthe reference patent is so designed that the sum frequency of opposedwires is kept constant. This feature is obtained by providing atransducer on at least one the wires. The transducer controls thetension of that particular wire.

Another patent of interest is U.S. Pat. No. 3,382,724, entitled 3-AxisAccelerometer, by Doyle E. Wilcox, of the inventors of the presentsensor. The device of that patent is a three-axis accelerometer whichutilizes a proof mass suspended by six string members, which stringmembers extend from the proof mass along the three axes. The stringmembers are whirled in a circular orbit, the frequency of which willvary as a function of the acceleration.

One of the main features of the present invention which is not taught inthe prior art is the means for keeping the tension on one wire constantwhile utilizing the other wire of a pair to provide a frequency signalwhich is equal to the sum of the forces acting on the proof body and thetension felt in the constant vibrating wire.

SUMMARY OF THE INVENTION The rate and/or acceleration sensor of thepresent invention is comprised of a reference frame and a proof bodywhich may be either a gyroscope or an accelerometer proof mass which ismounted to the reference frame by means of five pairs of string memberswhich support the gyroscope or proof mass along three orthogonal axes.Means are provided for exciting a natural frequency of vibration in thestring members. Pickoff means are used to sense the frequency ofvibration in each to remain constant at or near said natural frequencyin response to the signals from the pickoff means. Utilization means,for example, a computer, receive the pickoff signal from the firststring of each pair, which signals are proportional to the forces actingon said gyroscope or proof mass.

It is, therefore, an object of the present invention to provide a newand improved rate and/or acceleration sensor.

It is a further object of the present invention to provide a stringmounted proof body, capable of measuring rate and/or acceleration.

Still a further object of the present invention is the provision of anew and improved three-axis accelerometer, utilizing a proof mass.

The foregoing and other objects of the present invention will becomemore apparent and better understood when taken in conjunction with thefollowing description and accompanying drawings, throughout which likecharacters indicate like parts and which drawings form a part of thisapplication.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates in across-sectioned view a preferred embodiment of the invention;

FIG. 2 is a sectioned view of FIG. 1, taken along the section lines 22.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1wherein is disclosed a rate and/or acceleration sensor 9 in which agyroscope rotor 10 is suspended within a housing 13 by means of pairs ofstring members 21-22, 23-24, 25-26, 27-28, and 29-30. The string pairs23-24 and 25-26 are parallel to each other; with pairs 27-28 and 29-30parallel to each other. Each of the parallel pairs are mutuallyorthogonal with respect to each other and with respect to the pair21-22. The tension force each suspension wire exerts on the gyroscopewheel is sensed by exciting lowfrequency transverse vibrations in thewire and sensing the natural frequency of vibration. The naturalfrequency of transverse vibration of a tensioned wire or ligament isrelated to the tension force by the equation in this equation,

f frequency of vibration E modulus of elasticity of the wire Icross-sectional moment of inertia or wire m= mass of the wire l= lengthof the wire T= tension force The gyroscope rotor 10 is supported by thegas bearing 11 on shaft 12 within housing 13. The rotor is driven bymotor 14 mounted in the center of shaft 12. Power for motor 14 issupplied through a set of flexible leads connected between the housing13 and the structure 15. The housing 13 is suspended by means of theaforementioned five colinear pairs of wires. Wires 21 and 22 aredisposed along the Z or spin axis and constrain the gyroscope housing 13to the structure 15 along this axis. Wires 23 and 24 constrain one endof the gyroscope housing in the direction of the X-axis, and wires 25and 26 constrain the other end of the gyroscope along the X-axis. Thesefour wires, 23, 24, 25 and 26, also provide angular constrain of thegyro housing 13 about the Y-axisr Wires 27 and 28 constrain one end ofthe gyro housing 13 within the structure 15 in the direction of theY-axis and wires 29 and 30 (not shown) constrain the other end of thegyroscope within the structure 15 along the Y-axis. Wires 27, 28, 29 and30 also provide angular constrain of the housing 13 about the X-axis.

Although a gyroscope is shown as the proof body, it would be obvious tosubstitute an accelerometer proof mass for the proof body. Theaccelerometer proof mass would not be able to sense rate and, therefore,this function would be lost.

One wire of each wire pair has its outer end attached securely to amember providing some position adjustment, such as member 31. The outerend of the other wire comprising the pair is securely fastened to anelectromagnetic force transducer such as member 32.

Force transducer member 32 is comprised of a cantilever beam attached tostructure 15 by means of a thin flexure section (fulcrum) 33. One end ofthis beam carries a cylindrical permanent-magnetic field stator 34,commonly termed a potmagnet or loudspeaker field. The current-carryingarmature 35 is a cylindrically wound coil which is attached to thestructure l5, and engages the magnetic field of stator 34. With thisarrangement, an electric current flowing through armature 35 will exerta force on stator 34 proportional to the current and the magnet fieldintensity. Hence, by means of the leverage ratio of transducer member32, the force developed by armature 35 is increased as it is applied towire 24.

The lever member can be constructed with its center of mass located inthe region of the cylindrical permanent magnet field stator 34 on theopposite side of the flexure section 33 from the wire attachment point.The lever mass is adjusted such that an acceleration applied along theaxis of the wire causes a force component in the string which will beapproximately sufficient to balance the force accelerating the proofbody. With the mass adjusted accordingly, only a small electrical inputis needed to the armature 35 in order to hold a constant tension in thesecond wire, wire 23, of the wire pair.

Each pair of wires is pretensioned during assembly to a magnitudesufficient to keep each wire tightly stretched for all values ofacceleration and rate of turn experienced in normal operation. Thispretension is applied by elastic torque preset into the flexure member33.

Because the system of wires which suspend the gyroscope are elastic, theprecise location and orientation of the gyroscope axis within thestructure 15 will change when the structure is accelerated or rotated ininertial space. Such a shift in the spin axis will result incross-coupling and misalignment errors in the determination of theinertial forces and torques acting about the three orthogonal axes, X YZ. It is the purpose of this invention to provide a means for control oftension in the pairs of wire comprising the gyroscope suspension so asto effectively eliminate he elastic compliance of the suspension andthereby maintain a fixed orientation and location of the spin axiswithin the gyroscope structure for all conditions of acceleration andinertial angular rate.

To show how this is accomplished, the control of wire pairs 23 and 24will be described. These wires have an initial tension, T set into them.With this tension, wire 23 will vibrate at a frequency f in accordancewith equation (1). The transverse vibrations of wire 23 are excited bymeans of the signal from a capacitance pickoff 36 driving amplifier 37which powers the exciter coil 38. The magnetic field of coil 38 coerceswire 23 to vibrate transversely at its natural resonant frequency, f Theoutput signal from amplifier 37 having frequency f is also fed to thefrequency and phase comparator circuit 39 where it is compared with asignal having a precision reference frequency f,,. The output signalfrom comparator 39 is proportional to the phase difference between thereference frequency signal f and the wire vibration frequency signal fThis signal is fed to power amplifier 40 and the output current fromamplifier 40 passes through armature 35. This current flowing inarmature 35 produces a proportional force on tension transducer 32 byreaction of this current with the magnetic field of magnet 34.Transducer 32 multiplies this force by its leverage ratio, typically afactor of to 10, and applies this force as a tension modulation to wire24, which in turn controls the tension in wire 23. Hence, the tension inwire 23 is controlled so as to maintain its vibration frequency fconstant through comparison to the reference frequency f,,.

The constant tension in wire 23 implies a constant strain, or stretch,of wire 23. Hence, the end of the gyroscope connected to wire 23 willremain fixed relative to structure 35. All of the inertial forces alongthe X-axis applied by the end of the gyroscope housing 13 to thejunction of wires 23 and 24 are effectively carried by wire 24 as aresult of the servo action described. Vibration of wire 24 is effectedby means of the capacitance pickofi' and magnetic driving solenoid ofexciter 41 operating in conjunction with amplifier 42 to generate asignal of frequencyf The frequencyf of wire 24 will, therefore, beresponsive to the force F applied by the gyroscope housing 13 to wires23 and 24 in accordance with the equation sources which would otherwiseresult from elastic compliance of the wire suspension system underconditions of acceleration or inertial angular rate.

Referring specifically to FIG. 2, the torque reaction strut 50, which isconnected to the gyroscope housing 13 by means of flexure member 51 andthe outer structure 15 by means of flexure 52, will absorb the reactiontorque exerted on the housing 13 by the gyroscope rotor during startupand coastdown of the rotor. This start thereby prevents windup of thesuspension wires which would otherwise occur as a result of rotoracceleration or deceleration.

While there has been disclosed what is considered to be the preferredembodiment of the invention, it will be manifest that many changes andmodifications may be made therein without departing from the essentialspirit of the invention. It is intended, therefore, in the annexedclaims, to cover all such changes and modifications as fall within thetrue spirit of the invention.

We claim:

1. A sensor comprising in combination:

a. a reference frame;

b. proof body;

c. five pairs of string members supporting said proof body with respectto said reference frame along three orthogonal axes, each pair of stringmembers aligned along one of said three orthogonal axes;

d. means for exciting a reference frequency vibration in said stringmembers;

e. sensing means for sensing the frequency of vibration in a secondstring member of each of said pair of string members wherein saidfrequency of vibration is a function of the tension in each of saidsecond string members;

f. a reference frequency source g. transducer means interposed between afirst string of each one of said pairs of string members and saidreference frame for adjusting the tension in the string members of saidpair in response to the difference in frequency between the sensedsignal from said sensing means and said reference frequency to maintainthe frequency of vibration of said second string at said referencefrequency; and

h. pickoff means responsive to the frequency of vibration in said firststring of each pair of string members to provide output signalsproportional to the vibrations of said first string;

. utilization means for receiving the sensed signal from said picltoffmeans corresponding to the frequency of vibration of said first stringand providing therefrom a signal indicative of the forces acting on saidproof body.

2. The invention according to claim 1 wherein said transducer means iscomprised of:

a. lever members mounted to said reference frame by means of flexiblefulcrums, one end of said first string of a pair connected to one end ofsaid lever members;

b. electromagnet means attached to the other end of said lever membersto draw said lever end toward said reference frame in response to thesensed signal from said sensing means so as to adjust the tension incorresponding string members; and

c. said lever member having its center of mass located in the region ofsaid electromagnet means on the opposite side of said fulcrum from thestring attachment point, and having a magnitude of mass such that, withan acceleration applied along the axis of the string, the resultingacceleration force component in the string attached to the lever memberwill be approximately sufficient to balance the force necessary toaccelerate said proof mass, thereby requiring only a small electricalinput to said electromagnetic means in order to hold constant tension inthe second string of the pair. I

3. The invention according to claim 2 wherein said flexible fulcrumsexert coercive force on said lever members of a magnitude sufficient toestablish vibrations in said string members at a preselected frequencywith no forces acting on said gyroscope,

4. The invention according to claim 1 wherein said proof body is agyroscope.

5. The invention according to claim 4 and further comprisa rigid memberconnected between said gyroscope and said reference frame for limitingrotation of said gyroscope in response to rotations of the gyroscoperotor.

6. A sensor comprising in combination:

a. a reference frame;

b. a proof body, said body being a gyroscope having a rotor for rotationat high rates of speed about a spin axis;

c. a first and second parallel pair of string members attached at oneend to each end of said gyroscope along a first defined axis;

d. a third and fourth parallel pair of string members attached at oneend to each end of said gyroscope along a second defined axis orthogonalwith said first defined axis;

e. a fifth pair of string members attached at one end to each end ofsaid gyroscope parallel to the spin axis of said gyroscope and mutuallyorthogonal to said first and second defined axes;

f. a transducer means interposed between a first string of each of saidpairs of string members and said reference frame for controlling thetension in said string members;

g. sensing means responsive to the frequency of vibration in the secondstring of each pair of string members to provide a signal to saidtransducer means which is proportional to said vibrations so as tomaintain the vibrations in said second string of each of said pair ofstring members constant; and

h. pickoff means responsive to the frequency of vibration in the firststring of each pair of string members to provide output signalsproportional to the vibrations of said first string which signals areindicative of the forces acting on said gyroscope along the first andsecond defined axis and said spin axis.

7. The invention according to claim 6 wherein said transducer means iscomprised of:

a. lever members mounted to said reference frame by means of flexiblefulcrums, one end of each of said first string of a pair connected toone end of said lever members; and

b. electromagnet means attached to the other end of said lever membersto draw said lever end toward said reference frame in response to thesensed signal from said sensing means so as to adjust the tension incorresponding string members.

8. The invention according to claim 6 and further comprising:

means for comparing a reference signal against the pickoff signal fromsaid second string, and for providing a difference signal to saidtransducer means so as to vary the tension in said string pairs tominimize said difference signal.

9. The invention according to claim 6 and further comprising:

a rigid member connected between said gyroscope and said reference framefor limiting rotation of said gyroscope in response to rotations of saidgyroscope rotor.

10. The invention according to claim 6 wherein said flexible fulcrumsexert a coercive force on said lever member of a magnitude sufficient toestablish vibrations in said string members at a preselected frequencywith no forces acting on said gyroscope.

1. A sensor comprising in combination: a. a reference frame; b. proofbody; c. five pairs of string members supporting said proof body withrespect to said reference frame along three orthogonal axes, each pairof string members aligned along one of said three orthogonal axes; d.means for exciting a reference frequency vibration in said stringmembers; e. sensing means for sensing the frequency of vibration in asecond string member of each of said pair of string members wherein saidfrequency of vibration is a function of the tension in each of saidsecond string members; f. a reference frequency source g. transducermeans interposed between a first string of each one of said pairs ofstring members and said reference frame for adjusting the tension in thestring members oF said pair in response to the difference in frequencybetween the sensed signal from said sensing means and said referencefrequency to maintain the frequency of vibration of said second stringat said reference frequency; and h. pickoff means responsive to thefrequency of vibration in said first string of each pair of stringmembers to provide output signals proportional to the vibrations of saidfirst string; I. utilization means for receiving the sensed signal fromsaid pickoff means corresponding to the frequency of vibration of saidfirst string and providing therefrom a signal indicative of the forcesacting on said proof body.
 2. The invention according to claim 1 whereinsaid transducer means is comprised of: a. lever members mounted to saidreference frame by means of flexible fulcrums, one end of said firststring of a pair connected to one end of said lever members; b.electromagnet means attached to the other end of said lever members todraw said lever end toward said reference frame in response to thesensed signal from said sensing means so as to adjust the tension incorresponding string members; and c. said lever member having its centerof mass located in the region of said electromagnet means on theopposite side of said fulcrum from the string attachment point, andhaving a magnitude of mass such that, with an acceleration applied alongthe axis of the string, the resulting acceleration force component inthe string attached to the lever member will be approximately sufficientto balance the force necessary to accelerate said proof mass, therebyrequiring only a small electrical input to said electromagnetic means inorder to hold constant tension in the second string of the pair.
 3. Theinvention according to claim 2 wherein said flexible fulcrums exertcoercive force on said lever members of a magnitude sufficient toestablish vibrations in said string members at a preselected frequencywith no forces acting on said gyroscope.
 4. The invention according toclaim 1 wherein said proof body is a gyroscope.
 5. The inventionaccording to claim 4 and further comprising: a rigid member connectedbetween said gyroscope and said reference frame for limiting rotation ofsaid gyroscope in response to rotations of the gyroscope rotor.
 6. Asensor comprising in combination: a. a reference frame; b. a proof body,said body being a gyroscope having a rotor for rotation at high rates ofspeed about a spin axis; c. a first and second parallel pair of stringmembers attached at one end to each end of said gyroscope along a firstdefined axis; d. a third and fourth parallel pair of string membersattached at one end to each end of said gyroscope along a second definedaxis orthogonal with said first defined axis; e. a fifth pair of stringmembers attached at one end to each end of said gyroscope parallel tothe spin axis of said gyroscope and mutually orthogonal to said firstand second defined axes; f. a transducer means interposed between afirst string of each of said pairs of string members and said referenceframe for controlling the tension in said string members; g. sensingmeans responsive to the frequency of vibration in the second string ofeach pair of string members to provide a signal to said transducer meanswhich is proportional to said vibrations so as to maintain thevibrations in said second string of each of said pair of string membersconstant; and h. pickoff means responsive to the frequency of vibrationin the first string of each pair of string members to provide outputsignals proportional to the vibrations of said first string whichsignals are indicative of the forces acting on said gyroscope along thefirst and second defined axis and said spin axis.
 7. The inventionaccording to claim 6 wherein said transducer means is comprised of: a.lever members mounted to said reference frame by means of flexiblefulcrums, one end of each of said first string oF a pair connected toone end of said lever members; and b. electromagnet means attached tothe other end of said lever members to draw said lever end toward saidreference frame in response to the sensed signal from said sensing meansso as to adjust the tension in corresponding string members.
 8. Theinvention according to claim 6 and further comprising: means forcomparing a reference signal against the pickoff signal from said secondstring, and for providing a difference signal to said transducer meansso as to vary the tension in said string pairs to minimize saiddifference signal.
 9. The invention according to claim 6 and furthercomprising: a rigid member connected between said gyroscope and saidreference frame for limiting rotation of said gyroscope in response torotations of said gyroscope rotor.
 10. The invention according to claim6 wherein said flexible fulcrums exert a coercive force on said levermember of a magnitude sufficient to establish vibrations in said stringmembers at a preselected frequency with no forces acting on saidgyroscope.